A see-through, near-to-eye display (NED) unit may be used to display virtual imagery mixed with real-world objects in a physical environment. Such NED units include a light engine for generating an image, and an optical element which is partly transmissive and partly reflective. The optical element is transmissive to allow light from the outside world to reach the eye of an observer, and partly reflective to allow light from the light engine to reach the eye of the observer. The optical element may include diffractive optical elements (DOEs) or holograms within a planar waveguide to diffract the imagery from the microdisplay to the eye of the user.
In practice, NED units may include a stack of multiple waveguides, with each waveguide assigned to a wavelength component. In particular, by controlling aspects of the DOE within a waveguide, the waveguide may be matched, or optimized, to couple with the highest efficiency with a specific wavelength component. Optimizing the different DOEs to different colors of the visible light spectrum allows an NED unit to provide a full color experience.
In a stack of multiple waveguides, wavelength components matched distal waveguides in the stack (i.e., those furthest from the light engine) pass through the more proximal waveguides in the stack. Often the wavelength components intended for the distal waveguides couple into the more proximal waveguides instead of passing straight through. This results in loss of brightness, in non-uniformity of the colors reaching the observer from the NED unit and degradation of the quality of the reproduced virtual image.